Exhaust emission purification apparatus for an internal combustion engine

An exhaust emission gas purification apparatus for an internal combustion engine, comprising:

• a lean NO.sub.x catalyst arranged in the exhaust pipe of an internal combustion engine;
• a hydrocarbon supply means for supplying hydrocarbon into the exhaust gas for reducing NO.sub.x upstream of said NO.sub.x catalyst;
• a state variables detection means for detecting state variables representing the operating conditions of said lean NO.sub.x catalyst;
• a basic hydrocarbon supply amount determining means for determining the basic amount of hydrocarbon supplied from said hydrocarbon supply means in accordance with the state variables detected by said state variables detection means;
• a temperature difference detection means for detecting the difference between the exhaust gas temperature at the inlet of said lean NO.sub.x catalyst and the temperature of the lean NO.sub.x catalyst which are included in the state variables detected by said state variables detection means;
• and
• a hydrocarbon supply amount correction means for correcting the basic hydrocarbon amount determined by said basic hydrocarbon supply amount determining means in accordance with the temperature difference detected by said temperature difference detection means.